TypeError unhashable type list when using built-in set function

Question

I have a list containing multiple lists as its elements  eg    1 2 3 4   4 5 6 7     If I use the built in set function to remove duplicates from this list  I get the error  TypeError  unhashable type   list    The code I m using is  TopP   sorted set TopP  reverse True    Where TopP is a list just like in the e g  Above  Is this usage of set   wrong  Is there any other way in which I can sort the above list

User · Answer

python 3 2        gt  gt  gt  gt  from itertools import chain      gt  gt  gt  gt  eg sorted list set list chain  eg      reverse True           7  6  5  4  3  2  1             eg contain 2 list within a list  so if you want to use set   function    you should flatten the list like  1  2  3  4  4  5  6  7       gt  gt  gt  res  list chain  eg            1  2  3  4  4  5  6  7                         gt  gt  gt  res1  set res                          1  2  3  4  5  6  7      gt  gt  gt  res1  sorted res1 reverse True

User · Answer

Sets require their items to be hashable  Out of types predefined by Python only the immutable ones  such as strings  numbers  and tuples  are hashable  Mutable types  such as lists and dicts  are not hashable because a change of their contents would change the hash and break the lookup code   Since you re sorting the list anyway  just place the duplicate removal after the list is already sorted  This is easy to implement  doesn t increase algorithmic complexity of the operation  and doesn t require changing sublists to tuples   def uniq lst       last   object       for item in lst          if item    last              continue         yield item         last   item  def sort and deduplicate l       return list uniq sorted l  reverse True

User · Answer

Sets remove duplicate items  In order to do that  the item can t change while in the set  Lists can change after being created  and are termed  mutable   You cannot put mutable things in a set    Lists have an unmutable equivalent  called a  tuple   This is how you would write a piece of code that took a list of lists  removed duplicate lists  then sorted it in reverse   result   sorted set map tuple  my list    reverse True   Additional note  If a tuple contains a list  the tuple is still considered mutable    Some examples    gt  gt  gt  hash  tuple     3527539  gt  gt  gt  hash  dict      Traceback  most recent call last     File   lt pyshell 5 gt    line 1  in  lt module gt      hash  dict     TypeError  unhashable type   dict   gt  gt  gt  hash  list      Traceback  most recent call last     File   lt pyshell 6 gt    line 1  in  lt module gt      hash  list     TypeError  unhashable type   list

User · Answer

Definitely not the ideal solution  but it s easier for me to understand if I convert the list into tuples and then sort it   mylist     1 2 3 4   4 5 6 7   mylist2      for thing in mylist      thing   tuple thing      mylist2 append thing  set mylist2

[python] TypeError: unhashable type: 'list' when using built-in set function

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